1. Field of the Invention
This present invention relates to processes for the oxidation of dialkyl substituted benzenes to aromatic polycarboxylic acids. More particularly, the present invention relates to an improved method for recovering solvent that is used in processes for the oxidation of dialkyl substituted benzenes to aromatic polycarboxylic acids.
2. Description of the Prior Art
The liquid-phase oxidation of a polyalkyl substituted benzene to an aromatic polycarboxylic acid is well known. Such processes are described by Saffer, et al, in U.S. Pat. Nos. 2,833,816, 3,089,906, and 3,089,907, and Park, et al, in U.S. Pat. No. 4,053,506.
As shown in the Park, et al, patent, the condensed vapors from the reaction zone may be sent to a distillation column, the solvent dehydration column, to dehydrate the monocarboxylic acid solvent that is used in the reaction zone. The water of reaction, as well as water added to the process for scrubbing vent streams must be removed from the monocarboxylic acid, i.e., acetic acid. This water removal is accomplished by distillation in the solvent dehydration column.
Oxygen-containing gas, e.g., air, may be introduced into the first crystallizer of the product recovery system of a process for the oxidation of a polyalkyl substituted benzene to an aromatic polycarboxylic acid. As a result, this first crystallizer becomes a combination oxidation-crystallization zone, wherein both crystallization of product and secondary oxidation of reactant occur. Vapor, containing water of reaction, is removed from this oxidation-crystallization zone and is sent to the solvent dehydration column. This vapor, along with the condensed liquid from the reaction zone, is treated in the solvent dehydration column. This vapor stream contains acetic acid, water, and inert gas comprising nitrogen, unreacted oxygen, and carbon oxides.
The capacity of the solvent dehydration column may be limiting the production rate of the polycarboxylic acid. Therefore the rate of the oxygen-containing gas to the oxidation-crystallization zone may not be at the level which is required or suitable for efficient operation of the oxidation-crystallization zone, since the resulting vapor stream, when combined with the condensed liquid obtained from the primary oxidation zone, would overtax the capacity of the solvent dehydration column. Consequently, the solvent dehydration step would become a bottleneck to increasing production rates at an existing unit.
The present invention removes this bottleneck by shifting part of the water removal from the solvent dehydration column to a new tower or column, identified hereinafter as the "secondary dehydration tower" or "secondary dehydration column".